The field of the invention relates to air/fuel ratio control for motor vehicles having a fuel vapor recovery system coupled between the fuel supply system and the air/fuel intake of an internal combustion engine.
Modern engines are equipped with 3-way catalytic converters (NO.sub.X, CO, and HC) to minimize emissions. Efficient operation requires that the engine's air/fuel ratio be maintained within an operating window of the catalytic converter. For a typical converter, the desired air/fuel ratio is referred to as stoichiometry which is typically 14.7 lbs. air/lb. fuel. During steady-state engine operation, the desired air/fuel ratio is approached by an air/fuel ratio feedback control system responsive to an exhaust gas oxygen sensor. More specifically, a fuel charge is first determined for open loop operation by dividing a measurement of inducted airflow by the desired air/fuel ratio (such as 14.7). This open loop charge is then trimmed by a feedback correction factor responsive to an exhaust gas oxygen sensor. Electronically actuated fuel injectors are actuated in response to the trimmed fuel charge determination. In this manner, steady-state engine operation is maintained near the desired air/fuel ratio.
Air/fuel ratio control has been complicated, and in some cases made unachievable, by the addition of fuel vapor recovery systems. These systems store excess fuel vapors emitted from the fuel tank in a canister having activated charcoal or other hydrocarbon absorbing material to reduce emission of such vapors into the atmosphere. To replenish the canisters storage capacity, air is periodically purged through the canister, absorbing stored hydrocarbons, and the mixture of vapors and purged air inducted into the engine. Concurrently, vapors are inducted directly from the fuel tank into the engine.
Induction of rich fuel vapors creates at least two types of problems for air/fuel ratio control systems. Since there is a time delay for an air/fuel charge to propagate through the engine to the exhaust sensor, any perturbation in inducted airflow, such as caused by the sudden change in throttle position, will result in an air/fuel transient until the feedback loop responsive to the exhaust gas oxygen sensor is able to correct for such perturbation. Further, conventional air/fuel ratio feedback control systems have a limited range of authority. Induction of rich fuel vapors may exceed the feedback system's range of authority resulting in an unacceptable increase in emissions.
U.S. Pat. No. 4,715,340 has addressed some of the above problems. More specifically, a combined air/fuel ratio feedback control system and vapor purge system is disclosed. To reduce the air/fuel transient which may occur during rapid throttle changes, the purged rate of vapor flow is made proportional to the rate of inducted airflow. Allegedly, any change in inducted airflow will then be accompanied by a corresponding change in purged vapor flow such that the overall air/fuel ratio is not significantly perturbed during a change in throttle angle.
The inventors herein have recognized numerous disadvantages with the prior approaches. For example, modern aerodynamic styling has resulted in less air cooling flow around the fuel system and, accordingly, an increase in fuel vapor generation. In addition, government regulations are restricting the amount of vapors which may be discharged into the atmosphere. This trend will continue on an ever more strident basis in the future. Accordingly fuel vapor recovery systems in which purge flow is proportional to airflow may no longer be satisfactory because the rate of purge flow may be less than required to adequately reduce fuel vapors at conditions other than full throttle. The inventors herein have therefore sought to provide a system which inducts fuel vapors at a maximum rate over all engine operating conditions including idle. A need exists for such a system which does not exceed the air/fuel feedback system's range of authority and which does not introduce air/fuel transients during sudden throttle changes.